Community based studies have shown increased cardiovascular mortality associated with acute exposures to particulate air pollution. Electrocardiographic changes have also been reported in animals exposed to particles in controlled conditions. We have hypothesized that cardiovascular patients may experience life-threatening arrhythmias associated with particulate air pollution episodes. Implanted cardioverter defibrillator (ICD) devices continuously monitor the heart rhythm, and on detecting arrhythmias can initiate interventions. These devices provide a passive, continuous monitor of cardiac arrhythmias. We are assessing the association between community exposures to air pollution measured by ambient monitors and these cardiac arrhythmias detected by implanted cardioverter defibrillator devices.